The use of short-chained diols as chain extenders during the preparation of polyurethane plastics is known. Certain combinations of isocyanate and chain extender, of the type described, for example, in Kunststoff-Handbuch, Vol. VII, Vieweg-Hochtlen, Carl Hanser Verlag, Munich, 1966 on pages 206 through 213 or in High Polymers, Vol. VXI, "Polyurethanes, Chemistry and Technology", compiled by Saunders-Frisch, Interscience Publishers, New York, London, Parts I and II, have proven to be particularly suitable for the preparation of polyurethane plastics having excellent physical properties. Such combinations include the 1,5-diisocyanatonaphthalene/1,4-butane diol and 4,4'-diisocyanato diphenylmethane/hydroquinone-bis-(2-hydroxyethyl)-ether. Chain extension using 2,3-butane diol in conjunction with 1,5-diisocyanatonaphthalene also produces high quality plastics.
The high reactivity of the conventional chain extenders leads to very short pot lives, particularly in systems having a large rigid segment content, so that it is not always possible to process them into molded articles without experiencing difficulties. In addition, the large quantities of heat evolved during an excessively fast reaction with the chain extender can lead to the formation of inhomogeneities and cracks in the finished molded article, which, of course, have detrimental effects on the properties of the product.
It has now been found that by adopting known processes, polyurethane plastics having good physical properties and an adjustable pot life can be prepared if 1,4-3,6-dianhydro-hexites are used as chain extenders. It is surprising that the complete replacement of non-cyclic chain extenders in cast elastomer systems with 1,4-3,6-dianhydro-hexites leads to polyurethane plastics which are not only comparable in quality but are much better in some parameters, such as tear-propagation-resistance and structural strength. In view of the completely different structure of the chain extenders according to the present invention and the known sensitivity of polyurethane elastomers toward structural changes in the rigid segment, this result was not anticipated.
Moreover, it was also surprising that high quality polyurethane plastics could be produced by using mixtures of various 1,4-3,6-dianhydro-hexites in any proportions, either with each other or with other diols as chain extenders. Mixtures of extenders generally lead to inhomogeneities in the rigid segment and thus, to poorer physical cross-linking of the plastic and the consequent disadvantages with respect to quality.
An improvement advantage in terms of processing of the above-mentioned mixtures is that they are liquid at room temperature. This simplifies the application of the chain extender considerably as previous melting is omitted.